let arr = ['a', 'b', 'c', 'c', 'd', 'e'];


Array.prototype.my_includes = function (item, ...args) {
    let index
    if (args.length && !Number.isNaN(Number(args[0]))) {
      index = Number(args[0])
    } else {
      index = 0
    }
    for (let i = index; i < this.length; i++) {
      if (this[i] === item) {
        return true
      }
    }
    return false
  }
  
  const res = arr.my_includes('a')
  
  console.log(res);
